In accordance with a typical prior art stretching process, such as, for example, the SIGNODE process, a cast sheet of thermoplastic material, such as, for example, polypropylene, is first reduced in size, that is, the thickness dimension thereof, by rolling the sheet through a pair of closely spaced milling rollers or cylinders which rotate in opposite directions. After the thickness of the sheet has been reduced, the sheet is then drawn and stretched out from the milling rollers by means of a series of orienting rollers or a bridle assembly so as to achieve its finalized desired size or thickness.
Another prior art process or method that commonly used in connection with the fabrication of stretched sheet materials is a process known as or called the short-gap method and is generally comprised of an entry bridle, a stretching assembly, and an exit bridle. In accordance with this process, a slow speed, heated entry bridle assembly advances a cast sheet of material, usually film, to a stretching assembly which comprises a pair of rollers or cylinders which are set a predetermined distance apart. The first roller rotates at the same speed as the entry bridle, whereas the second roller rotates at a speed which is greater than that of the first roller and which is equal to the rotary speed of the exit bridle. Thus, as the film passes through the entire assembly, it is stretched to its finalized desired size or thickness dimension.
These prior art methods or processes have presented several disadvantages. For example, the properties of the straps produced by these methods or processes provide or exhibit limited increases in strength without significant decreases in other desired properties. In addition, substantial necking of the sheets occur as the sheets are stretched over the distance or space defined between the rollers.
U.S. patent application Ser. No. 07/958,803, now U.S. Pat. No. 5,387,388, disclosed a novel process and apparatus for achieving the simultaneous milling and stretching of such sheets whereby the aforenoted problems were minimized, however, other operational problems with such process and apparatus were discovered which likewise required rectification in order to achieve simultaneously milled and stretched sheets of thermoplastic material which exhibited other desirable properties. For example, one operational problem or deficiency which was discovered in connection with the fabrication or manufacture of the simultaneously milled and stretched sheets of thermoplastic material in accordance with the zero-gap process and apparatus set forth and described within U.S. Pat. No. 5,387,388 is that when the thermoplastic sheet is ready to be worked, that is, simultaneously milled and stretched, by being conducted through the nip defined between the oppositely rotating rollers of the zero-gap assembly, it was found that the opposite surfaces of the thermoplastic sheet exhibited different surface temperature values. This is because as the thermoplastic sheet is routed about the first or upstream roller of the zero-gap roller assembly, the inside surface of the thermoplastic sheet, that is, the surface of the sheet which is disposed in direct contact with the first or upstream roller of the roller assembly, will be heated by the first or upstream roller, or at least have its surface effectively prevented from being cooled by the ambient air because such surface is disposed in direct contact with the first or upstream roller and not exposed directly to the ambient air. On the contrary, the outer or exterior surface of the thermoplastic sheet, that is, the surface of the sheet which is not disposed in direct contact with the first or upstream roller, is effectively cooled as a result of being exposed to the ambient air. Such temperature differential between the aforenoted surfaces of the thermoplastic sheet leads to density differentials throughout the thermoplastic sheet which adversely affects the various properties of the processed sheet, such as, for example, the tensile strength, weldability and associated properties, and split resistance.
Another problem or deficiency which was discovered in connection with the aforenoted zero-gap simultaneous milling and stretching apparatus and method was that as a result of the processing of the thermoplastic sheet, the opposite edge portions of the sheet became thickened, or in other words, the sheet did not exhibit uniform thickness or flatness across the width thereof. Consequently, when the processed sheet is to be subsequently processed into thermoplastic strapping, the thickened edge portions cannot be used to fabricate such straps unless further processing is performed upon the sheet so as to effectively reduce the thickness dimension of the edge portion to the desired thickness dimension characteristic of useable strapping.
Accordingly, the invention embodied and disclosed within U.S. patent application Ser. No. 08/353,721, now U.S. Pat. No. 5,525,287, was directed toward an apparatus and method for producing an oriented plastic strap which was simultaneously milled and stretched as a result of a thermoplastic sheet workpiece being passed through a nip defined between the oppositely rotating rollers of the zero-gap assembly, and wherein, in particular, in order to overcome the aforenoted deficiency of the zero-gap simultaneous milling and stretching apparatus and method with respect to the thickened edge portions of the milled and stretched sheet, edge heaters were disposed along the opposite edge regions of the sheet workpiece prior to entry of the sheet workpiece into the nip defined between the zero-gap milling rollers. Such edge heating was found to significantly reduce the width of the thickened edge portions of the milled and stretched sheet, as measured inwardly from the edge portions of the sheet, such that an increased amount of useable sheet could be produced while concomitantly resulting in a reduced amount of sheeting which had to be previously trimmed and discarded.
Additional studies performed in connection with the strap profile and the effects upon the same by means of the disposition of the aforenoted edge heaters have revealed, however, that while such edge heaters have in fact reduced the thickness dimensions of the sheets at the extreme outer edge portions thereof, concave portions or concavities are nevertheless still present within regions immediately adjacent to the extreme outer edge portions of the sheets. The concave portions have depth or thickness dimensions which are somewhat less than the thickness dimension characteristic of the main central portion of the milled and stretched sheet. In addition, a transitional bump region is also present at the junction between the main central portion of the sheet and the concave side edge portions thereof, and such transitional bump regions have thickness dimensions which are greater than that of the main central portion of the sheet.
A need therefore exists for the development of apparatus, and a concomitant method, for producing oriented plastic sheets by Simultaneously milling and stretching the same wherein the produced sheets exhibit substantially uniform thickness dimensions across the entire width thereof from one edge portion to the opposite edge portion by effectively eliminating the aforenoted concavities and transitional bump regions.